An outstanding first effort! Use it as much as you can. Baring heavy surf it will do alright. All of Mike Olivers points are correct, He is called Mad Mike because of his proclivety of putting his body into ocean situations that Capt. Mavel, Batman and Superman would shy away from.
The baskets I built and sold 15 years or so ago were much cruder that your first effort. And had all the same faults when stacked up against a $40.00 Orvis product that took an ergomic engineer a year to design,
Build another with out the open mesh sides and rig it with a nyon web belt either thru the rear wall or make it a wrap arround the front of the box. Drill 4 holes , two on the left and two on the right, in line with each other and the width of the nylon web belt and plastic belt buckle that clicks together. Cut straigh lines vertically between the holes and lighty sand the slits to smooth out the friction sharp edges cause. The Orivs and discontinued LLBean sriping baskets have belly moulded curves on the body side. That is a more comfortablew option, but you can live without it if you are in reasonably decent shape.
Enjoy getting used to your personal product. It will serve the purpose for now. When you start challanging the rocky surf at Montauk in what some of us call "Combat Fishing", in October, buy the Orvis product.