First off - you need to decide your spec.
“Close to IP68” isn’t a spec. IP67 is most common for tools that are designed to be rugged, but not used underwater. IP68 is typically reserved for things like underwater cameras, things that must not only be water proof, but waterproof over a longer duration of time and at greater depths where the external pressure is much higher. (IP67 is only waterproof to 1 meter for 30 min, 68 means you can specify waterproof up to 30 meters, etc)
Since you described this as a construction tool, unless it is being used by underwater divers than IP67 is more than adequate for rain/puddles/buckets of liquid.
Without this turning into a full engineering tutorials, most products do one of the following:
-Seal the enclosure permanently with welding/goop (like Ray mentioned above) - obviously this is more risky because it prevents any rework at the manufacturing level, and the housing would need to be destroyed to disassemble the device. This is usually only done on electronics that are meant to be tossed at the end of their useful life or small subcomponents.
-Leave the enclosure mostly unsealed, but protect your electronics - this can be done depending on the design of the actual product. Sometimes it’s OK for the product to get water inside of it, as long as your sensitive parts are properly protected with a smaller sub enclosure. You can also hydrophobically coat sensitive parts to protect them. In a construction environment however, you probably have a lot of dust and particles to protect against
-Build a compression seal between your two housings and any openings of the product - this takes your standard O ring design and modifies it to fit your product. A soft rubber/silicone/elastomer will be squished between your two housings and compressed by the force of the screws that hold your product together. This requires the most engineering because obviously with a complex ID you will need to make sure that your screw bosses are consistent enough to apply a proper amount of force (but not to look ugly as shit) to compress the seal. You will also need to make sure the seal is easy to assemble - some products have very complex seals that slight misalignments during assembly can cause the seal to flop over, twist, and then not properly seal. This means that even though some of your products will be IP67, others will fail sealing tests in the field.
Anything you seal with a compression seal will require you to also seal any openings. Take a look at some of the earlier Samsung phones and you can see the very tiny seals that were used to seal the battery door, USB ports, etc. Those seals are not terribly robust, especially in a construction space where dirt on the seal will create openings for water to enter the device.
I’ve done a number of products where the seal material itself is over molded on to one of the housings using elastomer and eliminates the need for the O ring (and additional space it drives for the channel and wall thickness on each side).
You can see some of the seal designs on this image - the outer material of the top cap was TPE (though due to quality issues we had to add a third softer durometer material to improve the sealing consistency) and had foam gaskets and poron seals around the openings for things like the speaker and receiver. This was not a flawless design by any means - it took a great deal of energy and rework to get the rubber durometers correct and the multi angled seal on the keypad was a nightmare since it often shifted during assembly.
If you can get away with a flat, and consistently compressed seal your life will certainly be the easiest. But cell phone companies have taught us that there is a lot of clever engineering that can be done to seal a device that doesn’t require a 5mm thick wall section to achieve.
