Never heard of plasti- dip before... Both of my boas (hypo BCI and male BCA) are still small enough to be in plastic tubs. I put black contact paper on the back and sides of the larger tub for the BCA (it's a 110 quart), but the samller tub the BCI is in has nothing around the sides and he seems to do fine.

I would suggest using a soldering iron to add some ventilation holes along the sides of the tub (I put about 5 on the long sides and 3 on the short sides) and you can add more as you go if it is too humid.

I'd also suggest using luggage straps or bulldog / binder clips to help keep the lid of the tub secure. I have used both of these and find them easy to use and they make me feel a little better in terms of the snake being secure.

I would also suggest getting a branch or pvc or something that the boa will be able to perch on both of mine climb, but especially the smaller BCI (he is a baby at about 5 months old and roughly 18" long).

One last thing about the tubs.. I am not sure how much experience you have using them, but they don't hold temps well. I am keeping mine in a warm room now, but will be moving them into a heated snake room soon. In the past I had to use a heat lamp (CHE) on a dimmer switch, suspended about 12" above the tub on a lamp stand to get the correct ambient temp.
For the UTH, I use an ultratherm heat pad (these are sold by reptilebasics and won't melt your tub) on a thermostat for the hot spot.

I'm not sure how knowledgable you consider your reptile shop, but if you are not sure about their snake-sexing abilities, you might want to get a boa from a reptile show or breeder who can reliably sex it. With the females getting so much larger, it would really be worth it to make sure you are buying a male if you want a boa that will stay smaller.

Also, do some research on IBD (inclusion body disease) this is a common issue with boas, especially those that might have been around a lot of snakes (and therefore, at more risk to being exposed to it). I highly suggest quarantining any new snake you get, but IBD can lay dormant for a long time before showing symptoms.

I'll also add one more note. Be aware that boas, like BPs, change in color as they age. They get increasing brown pigment, so they won't stay as bright and light as the babies you've seen. Check out adult pics of the morphs you're interested in and if you want a lighter boa (as an adult) buy the lightest, cleanest baby you can find.. Albinos will also loose most of their pattern / contrast as they age. If you want a specific look, again, it may pay to find a good breeder so you can see pics of the parents.