I have a couple of mattress toppers that I'm planning to bring in a box structure duck taped up from the US.  It will meet the limits for the aiorline and I don't think my airport will fuss over it.  Will Customs in BKK make a fuss with it?  Anything I need to be aware of?  Thanks

I've brought a couple of boxes through in the past. One plain box like yours was ignored, the second time the officers on the x-ray machine asked what was in the box (it was actually what was shown on the outside, a PC monitor). They didn't even open or x-ray it, just waved me through.

 

With nothing untoward inside I doubt you will pique their interest.

 

Only time I have ever been stopped at the Green channel coming into Thailand and told to go to the Red channel was when I brought in a Hammock, with spreader bars,  that was in an over sized cardboard box

 

Customs Agent could not speak English so I mimed a person sleeping between to poles and after X'ray he let me pass through, none of my other three suitcases were touched or looked at  

I bring back an 18x18x18 box every trip (so 4 times per year) filled with personal items (clothing, magazines, electronics, foodstuffs, household supplies, etc.). In the green channel they always ask me to pop the box  - and no other luggage - onto the X-ray machine, and then I load it back on the trolley after. They've never asked to examine it.

 

In the U.S. my box gets opened 100% of the time in the departing airport by TSA, they leave a note-card informing me of the inspection. They re-tape the box.

I returned Monday morning (1/1/18), waved through with box. First time (out of ~ 60 times) they didn't ask me to put it through the X-ray machine.

 

Also the first time TSA didn't open/inspect my box departing the U.S. Maybe my 'Trusted Traveler' details are in their database?