There are a number of ways to have a flat fixed seat on your rowing machine. From the simple duct taping it in place to the WinTech adaptive seat with removeable back (see photo below). The latter costs hundred while the former costs pennies. But an excellent basic option are erg monorail clamps (photo above). While not listed in their catalogue, the clamps are sold by Indoor Sport Services in the UK. You can contact them directly (easiest to do with their Messenger app on their website). The cost is £22.50 per clamp plus shipping at £5.00. Thanks Nikki Stanton-Ketley on the Marlow adaptive squad who is an AR2 and uses them regularly for her training and says they work superbly well.