The OEM product is a 250 mm long x 15mm wide piece of stainless steel tubing with a threaded insert pressed into each end to accept the two 8 mm bolts which secure it to the two crash bar brackets.
There are two ways to DIY this, but note that you do this entirely at your own risk!
If you have a couple of 8mm stainless bolts, and are able to drill and tap an 8 mm thread into each end of a piece of stainless bar, then I would suggest this 12mm wide x 250 mm long type:
https://www.ebay.co.uk/itm/Stainles...ALL-SIZE-METALS/282567223835?var=581733076353
Cost of bar £10.56
If you are not able to do this then the alternative would be to use a suitably sized piece of stainless steel tube. To secure this you could run a piece of M8 threaded stainless steel rod through the tube and cut to length such that it projects far enough out of each crash bar bracket to accept a stainless steel locking nut. To tidy it up a black plastic nut cover could be put over the nut on each side. It might be necessary to cut a slot in one end of the threaded rod to allow the use of a flat bladed screwdriver to stop the rod from rotating while doing up the nuts. If the tube is a little loose even after tightening the nuts, then a stainless washer could be added at each end between tube and bracket as a spacer.
(Tip: put an M8 nut on the section of rod you are keeping, before cutting off the excess. Removing this nut will then clean up the threads damaged by the cutting, making it easier to put the locking nut on.)
I already have the nuts, plastic nut covers and washers, but have added those to the following list of bits in case you don't:
250 mm tube:
https://www.ebay.co.uk/itm/1pc-Silv...ube-12mm-OD-10mm-ID-250mm-Length/392266172886
300 mm x M8 threaded rod:
https://www.ebay.co.uk/itm/M8-A2-St...-Allthread-Stud/272080459684?var=570890403786
Stainless M8 nuts:
https://www.ebay.co.uk/itm/M2-M2-5-...YLOCK-LOCK-NUTS/320901621860?var=510063390508
Nut covers:
https://www.ebay.co.uk/itm/M5-M6-M8...-Caps-Cover-Hex/161319324291?var=460349179708
Stainless M8 washers:
https://www.ebay.co.uk/itm/M4-M5-M6...-FORM-C-WASHERS/221017647548?var=520086449280
Total cost of all bits: £12.51
Users of this forum don't seem that keen on DIY generally, but I hope that is useful to someone!