There are a few universal solutions that you could look at.
I see Cyberpower have a 4-post rail solution on amazon. https://www.amazon.ca/CyberPower-4POSTRAIL-4-Post-Universal-Silver/dp/B00TCELZTK/
I am also a fan of these super generic APC rails. If you need, you can knock out the nuts to bolt them more securely. Just look for APC 756H as there are a few different versions.
I had the same issue with my ups as my rack is too shallow for proper sliding rails.
Went with something like this and it works fine. Not pretty but just put your shelf on it and your UPS shouldn't move at all.
Maybe do the same for the PC to lay it down on one side so all the connections are at the back? Might look a bit better. Also you could add a keystone panel on the back where you feed the cables in - that way you can plug/unplug stuff without crawling into the rack. At least if the cable slot on top is big enough to get your hands in ;)
The rails aren't the ball bearing kind, so the UPS doesn't slide easily, so you could also just get some generic L-shaped mounting brackets instead.
I mean if you're just stacking servers on the bottom you don't even need a rack, if you're mounting the ears to the rails it may be okay if the bottom is perfectly flush but it likely isn't.
You need rails or a cheaper more generic option like rail shelves to equally distribute the weight. Most stuff beyond switches are not designed to be mounted by the ears alone, the server will sag, the rack will have unnecessary stress on front rails of the rack, it will become front heavy, etc.
If you can't find sliding rails to fit your equipment or just desire something quick and easy you can use something like these..
You lose 1 notch of the U immediately below it as it hangs a bit but it will do the job of equally distributing the weight and you can easily slide the equipment in and out.
Worse case scenario they make rails that are universal in the sense that they provide lips on both sides that act as a shelf.
I have used those. They aren't perfect as they hang into the next U by a notch so you have to keep a notch space between equipment using them but they work.
I have a variety of shelves for different purposes.
Or do you mean the rails?
I highly prefer these: https://www.amazon.com/gp/product/B00TCELZTK/
They are length adjustable, and most importantly, have very flat head rack mount screws and 3 mount holes (in a 1u format). This lets me attach stuff on top of the rail, flush, and without needing to compromise the integrity of the rails by removing important mount screws to remove hardware.
I couldn’t find any rails that worked for the Hyve or Synology’s in a short depth rack. So I purchased these rails below and they worked wonderfully
https://www.amazon.com/dp/B00TCELZTK?ref=ppx_pop_mob_ap_share
This is what I used. Only $36 and it's adjustable in length. https://www.amazon.com/dp/B00TCELZTK/ref=cm_sw_r_apan_i_G36QZYQEN00QGJH1XT8Y?_encoding=UTF8&psc=1
by other side I meant what I put in the rack.
So if I understand correctly, I have a of these on the server's case and I make them slide into a pair of those which is the part that always stays in the rack?
I'm having an issue understanding how do I screw in the front of the server to the vertical rack once I use those holes to secure the horizontal rails?
In this picture, the horizontal rails are using the same holes as the gear above it for instance
I used this rail kit: https://www.amazon.com/gp/product/B00TCELZTK/
Mine didn’t have rail screw holes so I had to get the shelf
CyberPower 4POSTRAIL 4-Post Universal Rack Mount Rail Kit, Silver https://www.amazon.com/dp/B00TCELZTK/ref=cm_sw_r_cp_api_glc_fabc_-pjdGbCSBZXBH?_encoding=UTF8&psc=1