Hey TinPusher
So, If you have no experience with 3d modelling programs, I'm not sure there is really a fast and easy way to create even simple objects. If you have the time, I would certainly recommend picking up some 3d modelling/texturing skills. If you have money to invest, 3ds max is an excellent program, and for free you can get Blender. Both are quite intimidating to a person who has no 3D experience.
Perhaps Google Sketchup would be the easiest I've seen, but I've never used it. FPSC uses .x file format for it's models, so with Sketchup-free, you can export to .dea Collada format only, which means you would have to convert it to .x format. Fragmotion is a good program to use with FPSC and you can try it free, if you are willing to say a little prayer.

(it can export to .x)
Modelling a box involves, creating a box,(easy), then you have to do a UV unwrap. UV's are texture co-ordinates. Imagine taking your box and cutting it up with scissors to lay it flat. You then have to assign each side of the box to the area of the texture you want to appear. Even a simple box can be tricky for a newbie, but like I said, it well worth it to pick up the skills if you invest the time.
Alternatively, if you just want to get your models in the game as quick as possible, I would recommend just retexturing the existing FPSC models. Most models use a .dds texture format, so you will need a program that can handle them. I like GIMP, it's free, but you may need a .dds plug-in to get it to work.(not sure if it comes pre-loaded or not.) You could also use Photoshop if you have that, but most likely will need a plug-in also. (note.) You can also use .tga format, and FPSC will auto convert it to .dds, also .jpg but I have no experience with this format in FPSC.
The good thing about using existing models, is the UV's are already mapped. Just open the image texture from the model you would like to replace,(lots of crates in Sci-fi entity section) and create your image the same size as the original. eg... 512x512. make sure your elements are in the same place as the original,eg...sides of the box, and replace the original texture with yours. I would recommend saving the original as a backup. I just simply add 'bu' to the file name, such as cratebu.dds. You could also just keep the original named as it is, save your texture with a different name, and the edit the entity's .fpe file, pointing it to your texture. eg...texturebank\user\mycrate.dds.
Hope that's not to confusing

Feel free to ask any questions if you need clarification with anything.

Oh ya, if you do make your own model, you will need a .fpe entity file and a thumbnail image. I recommend the Entity Workshop,by Wehtam, which creates a basic .fpe and thumbnail for you.