Knock one together out of pine...stain and seal. Glue and nail. Hammer, nails, saw...that's all you need.
Buy "select" stuff that's already dimensioned and finished (don't think you have the setup to finish lumber properly)
Simple V-shaped cutouts work (and are easy to do). A plain board with pegs works too.
Points to consider:
- deep enough so that when your rifle + optic is in there the scope isn't hanging out the back...you'll end up with the rack against the wall at one point and bang your scope against the wall otherwise
- the top board that catches the rifles is at an average height....too high and shorter CQB stuff might not sit securely
- consider an additional lower board to hold sub-guns
- glue plain craft felt onto the surfaces that'll touch the guns (green and red are the traditional colours...jk made that up
- chamfer any sharp edges or else nicks/bumps will show up more (best done with a plane...but you can use sandpaper on a wood block)
- use a framing sqare
- allow for adjustable feet (you can get them at the hardware store)...you might build it perfectly square...but your floors/walls probably aren't
If it's going pretty much permanently against a wall....you can make a top board and a separate bottom board and just secure those to the wall/floor respectively. Same if it's going into a makeshift cabinet/closet.
....after all that....they sell ready made ikea like kits at gun stores...I think Al Flaherty's had some.