It's been surprisingly difficult to find what I need. I did discover one model from a wargaming company a long time ago, but I felt it wasn't suitable for me. I can't even remember why that was; perhaps it was the wrong scale, or the international postage was too high. Maybe it was some other reason.
Then I found, almost by accident, that there is a huge variety of moulds of different objects sold on eBay. These are intended primarily for cake decorating, but a silicone mould can be used for a wide range of materials as well as for chocolate!
Most of the moulds I found are of little use to me (flowers, birds, scrollwork and the like) but I did find one for a Celtic Cross. So, I spent the necessary £3-and-a-bit and the mould was with me in just a couple of days.
So, I cast 2 crosses from the mould. These are quite thin; I smoothed off the backs with some coarse sandpaper. The 2 parts were then glued together, back to back. I needed to add a little filler around the seam, but that wasn't too hard.
So there it is: a Celtic Cross suitable for use with 28mm figures. I don't think it would look amiss with 20mm or 15mm figures either, though obviously it would represent a correspondingly bigger monument.
I shall probably use it as an objective in the SAGA "Sacred Ground" scenario, or maybe just as scatter scenery. If/when I ever get round to making my Celtic monastery (Scheltrum Miniatures has just the right buildings!) then this will fit in perfectly. And of course I can make as many more of these as I want, any time!