Question Applying image effects at lower resolutions to the render texture

Hello!

I've been putting together a very expensive image effect to render volumetric fog with ray marching.
I was wondering if anyone more experienced with image effects knew how to render the image effect at a lower resolution to the render texture resolution (e.g overlay 1080p volumetric image effect over 4K render texture).

Here is what my code to apply my vert/frag shader currently reads as:

Code (CSharp):

1.     [ImageEffectOpaque]
2.     private void OnRenderImage (RenderTexture src, RenderTexture dest) {
3.         if (material == null) material = new Material (shader);
4. #if !EDITOR_PLAY_MODE
5.         // assign material properties //
6. #endif
7.         Graphics.Blit (src, dest, material);
8.     }

The shader itself effectively just multiplies the pixel by a float for transmittance and adds a float3 for the light that scatters in the volume

Abbreviated below:

Code (CSharp):

1.        Cull Off ZWrite Off ZTest Always
2.  
3.        Pass
4.        {
5.            CGPROGRAM
6.  
7.            #pragma vertex vert
8.            #pragma fragment frag
9.  
10.            #include "UnityCG.cginc"
11.  
12.            struct appdata {
13.                float4 vertex : POSITION;
14.                float2 uv : TEXCOORD0;
15.            };
16.  
17.            struct v2f {
18.                float4 pos : SV_POSITION;
19.                float2 uv : TEXCOORD0;
20.                float3 viewVector : TEXCOORD1;
21.            };
22.          
23.            v2f vert (appdata v)
24.                v2f output;
25.                output.pos = UnityObjectToClipPos(v.vertex);
26.                output.uv = v.uv;
27.            
28.                float3 viewVector = mul(unity_CameraInvProjection, float4(v.uv * 2 - 1, 0, -1));
29.                output.viewVector = mul(unity_CameraToWorld, float4(viewVector,0));
30.                return output;
31.            }          
32.  
33.            float4 frag (v2f i) : SV_Target
34.            {
35.                // raymarch code to calculate transmittance and volumeCol //
36.                   (uses depth texture)
37.              
38.                float3 baseCol = tex2D(_MainTex, i.uv);
39.                float3 col = baseCol * transmittance + volumeCol;
40.                return float4(col,0);
41.            }
42.  
43.            ENDCG
44.        }

Any tips on how to approach this problem would be much appreciated