DIY rack without ever having used or seen a real one... oh my.
OK.
Whether in $, €, or £, the contents of a professionally used rack start in the four digit range, and a filled rack will have contents valued in the five digit range - for the hardware alone. Usually, them properly functioning will be worth even more.
For that reason, professional racks are made sturdy enough that hardware doesn't come crashing down on other hardware. Some expertise went into that.
You give no parameters about what you are going to do - but rule of thumb: No-one uses anything but solid steel for racks. Reason, see #1.
If you can use rails, use them. It's safer. (see #1, again...)
No guarantees for any damage you may cause - but... rackmount items that are mounted on one side only sometimes have a weight up to 15kg. That is, with 2-3HE, not a single 1HE mount! So, it's possible - if the vertical part is made by professionals who know to pick the right material and thickness.
For a selfmade thingy without any experience, I recommend (a) definitely using a back part to the frame, (b) using rails for every single mount in there. And make really, really sure the frame can hold several hundred kg without bending or twisting.
DIY racks are IMHO entertainment for seasoned rack users who know a lot more than I do... if you think you can save money this way, think also about the damage it might do if it fails. Professional racks aren't that expensive.
YMMV, just my 0.02€, no guarantees, usual disclaimers apply.