Do you have any woodworking skills? I am not sure I would waste time on cardboard or just whip up a box with 2x4 and plywood, though with the price of wood these days, that may cost more than the amp.
https://www.packagingsupplies.com/collections/wood-shipping-crates
https://www.uline.com/BL_427/Standard-Wood-Crates
Don’t waste your time with regular packing foam or anything like that. Get expanding foam bags. They will conform exactly to the amp and box. Big amp like that, you will need 4 or 6 bags: https://www.amazon.com/EcoBox-Inches-Instapak-Temperature-2032-1/dp/B00KAFU2SK/ref=sr_1_15?dchild=1&...
Is it >4 hours each way? That is 16 hours total. 1000 miles. There is a very real cost for your vehicle usage, probably more than shipping.
https://www.packagingsupplies.com/collections/wood-shipping-crates
https://www.uline.com/BL_427/Standard-Wood-Crates
Don’t waste your time with regular packing foam or anything like that. Get expanding foam bags. They will conform exactly to the amp and box. Big amp like that, you will need 4 or 6 bags: https://www.amazon.com/EcoBox-Inches-Instapak-Temperature-2032-1/dp/B00KAFU2SK/ref=sr_1_15?dchild=1&...
Is it >4 hours each way? That is 16 hours total. 1000 miles. There is a very real cost for your vehicle usage, probably more than shipping.