One algebra of double cosets for a general linear group over a finite field

Let $\mathbb {F}_q$ be finite field with $q$ elements. Let $α\leqslant n$ be positive integers. Consider the general linear group $\mathrm{GL}(α+n, \mathbb {F}_q) $ and its subgroup $H(n)$, which fixes the first $α$ basis elements in $\mathbb {F}_q^{α+n}$. Denote $\mathcal{A}_n$ by the convolution algebra of $H(n)$-biinvariant functions on $\mathrm{GL}(α+n, \mathbb {F}_q) $. We describe algebras $\mathcal{A}_n$ in terms of generators and relations and show that the family $\mathcal{A}_n$ admits a natural interpolation to arbitrary complex $n$ (the field $\mathbb {F}_q$ and $α$ are fixed).