Ikea's Kallax cubes are the gold standard for affordable vinyl record storage. When my 4×4 started to fill up, I found that I had just enough room to stack a 2×4 on top. That will hopefully be enough space for a few more years.
The Kallax shelves are about 15″ deep. Since LPs are about 12″, that leaves some extra space. For the lower shelves, I'm fine with pushing the records all the way back. It makes them safer from vacuum cleaners and other household dangers while remaining accessible. But records pushed back on the top shelves are too hard to see and to reach.
This is not an unusual problem. The intertubes are full of “Kallax backspacers“, both commercial and DIY. One cheap and easy suggestion I found was a foam pool noodle. They're not only easy to find and work with, but they're soft enough to keep from damaging your record sleeves. I didn't have a pool noodle readily available, but I did have some leftover foam pipe insulation.
I cut a section to the appropriate length, then shoved it into the back of one of the top cubes.
Voilà! The top-shelf records are now all visible and easily accessible. Cost? You can get a 6' chunk, enough for 5 cubes, for under $2.