Based on this product's specifications, here are some alternatives and similar models
Basil trunk bag MILES TARPAULIN TRUNKBAG XL PRO 9–36L, 100% waterproof, black lime (no mounting plate)
Summary description:
Trunk bag Basil MILES TARPAULIN TRUNKBAG XL PRO is a 100% waterproof pannier-style bag with a variable capacity of 9–36 liters. Designed for mounting on rear racks, it offers weatherproof storage for commuting, touring or shopping. Supplied without a mounting plate; material and exact attachment method follow manufacturer specifications.