Staring at the price of the Sevenlife 18 latex mattress in the checkout window, I decided to hone in my Google-Fu to further uncover any additional information and options. The first thing I found was an Amazon listing, and while the vast majority of reviews were positive, there was one 1-star review (from almost 6 years ago) that stood out:

My whole justification for such a £600 mattress was that it would outlast other options. A 10-year warranty is meaningless if it's financially infeasible to make a claim.

Understanding that the main issue is overseas shipping of a large and heavy item, I tried a few searches in both metric and imperial measurements and found a company called Purelybeds selling mattresses in 1/2 ft increments up to 7.5 feet (228.6 cm):

Because I'd have some spare slats from the headboards, in theory, I could spend about £10 on a staple gun to staple the extra 2 - 3 slats for their longest option.

I continued to look for "7 ft" and longer mattresses and found Bed Cloud, a company that makes mattresses in 1/4 ft increments up to 7 feet at prices much closer to those found on Amazon:

The first one I clicked on was the one with the grid pattern because I liked the look, and it just so happened to be their 18cm soft/medium offering which is very comparable to the Sevenlife 18 with the main difference being open-coil vs latex.Since the monitor is being mounted on the frame instead of standing on a pane of glass on-top of the mattress, I believe the non-localised nature of open-coil shouldn't be an issue.

Looking through the others shows a trend that higher-cost mattresses are firmer. I was able to find reviews on Amazon and, like the Sevenlife, most of them are positive. One gave a 4-star because it was softer than they'd like, which sounds good for me since I feel that "medium" mattresses are too hard.

It took a bit of effort to rework the frame to use a Three Way Outlet instead of a Three Way Through to be able to shave off the 7 cm I needed, in part due to Fusion sketch solving issues I had to track down and resolve. There is a minimum 100mm tube cutting length, so that's why there's a vertical 75mm and horizontal 50mm tube instead of them being shorter. 

The price of the frame increases by around £12, now £549, but the mattress savings can entirely pay for a new monitor and still have hundreds of pounds left over.