I suppose you mean Hat Lek in the Trat area near the coast, Khmer side calls Cham Yeam near Koh Kong ??? Its a straight forward border,in fact they are all similar crossings, just avoid Poipet. And try to exit and enter Thailand at the same point, it eases some Thai documents for a foreigner car (as mine was belgian), no idea if you have Thai numbers on it. I was years ago near Anlong Veng where Pol Pots last stronghold was on the Thai Border, forget the real name, but maps saYs CHARAT, it looked very quiet overthere when i was at the border, but i didnt cross because had a khmer rented car with me. To temporarily import a foreign car, the customs have a website were you have to register everything, and for confirmation you need a khmer phone number. At Koh Kong there are some fixers who can help you with all that hassle for a couple of dollars. It worked prettY bad on my laptop, and an official later told me it works perfect on a smartphone. Crossing the border there took me about 3 hours, wherefrom probably 1.50 for the problem with that website. As you bring goods with you, pretty sure you will have plenty of time to do all those things wile waiting the customs clearing from your goods, and probably need an customs-agent for that !!
https://customs.gov.kh/en is their website wich also has a lot of info for personal items.

On the webiste, the temporarily vehicle section is well hidden:
click on laws an regulations, then on the right from your screen you see E-services, and 6 lines down you can enter the temporary vehicle section. First you have to register yourself and make an account. For that you need a khmer phone-number where they send you some code (ofcourse 99% of foreigners dont have a khmer number before entering Cambodia. I did have one. The fixers at the border try to sell you a sim-card for ofcourse a special rate or give you their number to send the code !!