For my tuppence ha'penny worth Dave, I converted my box trailer (with removable mesh side extension sections and loading ramp) into a single bike trailer - whilst maintaining my ability to still use it as a box trailer (probably not a requirement in your case).
I bought a "Erde" 2m long heavy duty motorcycle carrying channel, which came with a bolt on hoop to locate the front wheel, but I replaced the hoop with a more substantial front wheel holder (wheel stow). No welding required - drilled though the plyboard/GRP box trailer floor and secured the channel to the floor and trailer subframe. The wheel stow attached the front of the channel and secured to the trailer drawbar with a U bolt arrangement. Due to the length of the trailer versus the length of a K100, it was necessary to take the angle grinder to the front of the box to chop out a section wide enough to take the front wheel. I kept the cut out section and attached it to the front mesh panel, so that it slots over the bike channel and wheel stow and doesn't leave a gapping hole in the front of the trailer when being used as a box trailer. I removed the trailer jockey wheels as it interfered with the wheel stow arrangement and, besides, I wouldn't want to be separating the trailer from the car with a bike loaded anyway...just in case things went horribly wrong!
Cost of the modification was about £150 - with hind sight, I could have found a cheaper option for the wheel channel rather than the proper "Erde" one...but it is a pretty substantial bit of metal work and does help spread the weight of the bike over a greater area. The wheel stow is invaluable - I can load my LT single handed as, once the front wheel locates in it, it holds the bike securely upright (without side or centre stand) while I fiddle around attaching ratchet straps front and back. I bought a special strap for the front - two webbing sleeves fit over the handlebars grips (cost £20) and makes the whole thing easier to strap down.
As shown below, I added a few additional straps - because I could and getting Gretel from Dublin to my home was my first experience of using my set up....and I didn't want any unplanned bike lie downs half way home!
My original concept was that I could use the trailers' loading ramp to get the bike on and off the trailer, but I soon discovered that the ramp was simply too steep to get a K100 up it easily. I nearly made it with some engine assist, but there was no grip on the channel section, so with the rear wheel still on the ramp, the front wheel would simply slide backwards as the whole ish returned back to the road. I overcame this by buying an additional folding motorcycle loading ramp which is much longer than the trailer ramp and therefore presents a much more gentle angle to be able to drive the bike up onto the trailer. Once the bike is loaded, the ramp simply folds up and is stashed in the bottom of the trailer (or back of the car). I think the loading ramp stung me another £40, but against the cost of dropping a K100/K75/K1 at the most in-opportune moment, probably a good investment.
In the ideal world, of course, I would opt for a proper drop axle trailer like Ed suggests - anything that makes loading easy, has got to be a good thing, but am too tight to fork out the £££ for something I will use infrequently!
Good luck with your project - it is reassuring to have a bike trailer to hand and perfect for dealing with outbreaks of KPox (you can't always do a Holister and ride home from the other side of the country...even on a relatively small island like the UK)!