Unfortunately every single window in the condo i rent is a horizontal slide. I've got a standard window AC unit and I've seen videos on building a frame to use it in a horizontal window, but has anyone here done it? Any tips or tricks/things to be concerned about? The ones meant for those windows are absurdly expensive and the central AC is garbage and never cools the 2nd floor.

Thanks all in advance

I resorted to taking the window slides out.
Then using 3/4" ply wood to fill the opening.
Then cutting an opening to fit the ac in the plywood. The portion you cut out can be used to make a shelf to hold the weight of the ac unit.

Will a stand up AC with a vent to the window work for you? I have one in my daughter's bedroom because HOA rules forbit window units sticking out.

like these:
https://www.google.com/search?...1-d&q=stand+up+a%2Fc

It is not ideal and depends on how handy you are. But yes a piece of siding or plywood type material to put above the unit is what works. Your ac unit will come with some necessary accessories to help accomplish the task. It is doable. Have some insulation handy to help seal up any openings or area's that help to keep heat out. Best of luck with it. Again, it is doable and will work to cool a room or maybe two.

Son lived in SF in a condo with same window type. We bought him an inside house unit that vents outside (like the link above) and had a piece of plexiglass cut to fill the gap. Worked well for 3 rooms.

I built a window replacement out of 2x2's, plywood, and insulation foam board. I made it the same size as the window it replaced; with my windows, you slide the window pane section to the center of the window opening, then lift up against the spring-loaded upper channels to lift the window pane section out. The insert works the same way. The size of the AC is limited by how much window space is lost to the insert.

I didn't have a router, so created the channels for the window slides by chisel. That was the most time-consuming part. Make the channels oversize, to make installation/removal easier.

I built it with a particular AC in mind. so made the hole for the AC narrow enough that the AC's "wings" were not needed.

The height of the AC gap has to be "Goldilocks" - large enough that the AC fits through it, small enough that the AC won't topple out. I added an extra lip along the outside of the bottom of the AC opening, to ensure I didn't lose the AC out the window (on the 2nd floor... could have been exciting).

I used soft foam with adhesive backing for insulation around the AC/insert gap, but it turned out to be hard to find around here. After the first year, I just used slender foam tucked into the gap.

I painted it to make it as un-ugly as possible; it is still ugly.

A problem: mud-daubers periodically take advantage of the sheltered area inside the window frame to build nests. All you can do is kill them as they show up.

Yep, the plywood cut to fit the window opening with a hole sized to fit the A/C unit is the way to go. I have done this with or without pulling the slider tracks, pulling them, and using ell brackets for the plywood mount seems the cleanest way to do it. Either fiberglass rolled and tamped into place or foaming insulation to seal the inevitable cracks.