Well, you actually have two problems.

The rear is curved, and they have rear ports, so they wont work well at all in that location. You need to look at some other speakers.

You need to get speakers that are either front-ported or sealed for use in a cabinet.

I know the Pioneer speakers are low-priced and have good reviews, but the 3.5" driver is too small to give any bass at all. That is another problem.

The KEF C1 speakers are much better speakers which will give some decent bass, are front-ported, can be placed on their sides (6" high) and can be purchased at KEF Direct for $279 right now.

The KEF C3 is another excellent front-ported speaker, and is on sale at KEF Direct for only $199 (regularly $349). It is also a good one for you, but it is slightly larger (7" high) , so check the dimensions carefully. That would be my first choice if it isn't too big, and the price is a real steal!