Check out Autokicker on Amazon , it's a lesser known brand without the premium price tag . I've got a small tailpack which straps around under the seat ,It;s really well made and IMHO is a cracking product , not got any experience of their magnetic tank bags , though it might be worth a look .
https://www.amazon.co.uk/s?k=autokic...b_sb_ss_i_3_11
Not Magnetic , the strap on version https://www.amazon.co.uk/Autokicker-...s%2C155&sr=8-7